A typical polarization maintaining optical fiber includes a pair of stress applying regions on both sides of a core region within a cladding region. In this polarization maintaining optical fiber, the residual stress component in the first direction connecting the respective centers of the pair of stress applying regions differ the residual stress component in the second direction being orthogonal thereto, thereby the optical fiber has birefringence. Accordingly, it is possible to propagate light while maintaining a polarized state of the light.
An example of a method of manufacturing this conventional polarization maintaining optical fiber will be described below. A cladding portion made of silica glass is formed on an outer periphery around a cylindrical core portion made of GeO2 doped silica glass. Then, an intermediate is formed by providing a pair of cylindrical holes within the cladding portion in parallel along the axial direction on both sides of the core portion. A cylindrical stress applying part made of B2O3 doped silica glass is inserted into each of the pair of holes on the intermediate, and by drawing from these materials and integrating them together, a polarization maintaining optical fiber is obtained.
According to this drawing, the core portion of the intermediate becomes the core region of the polarization maintaining optical fiber, the cladding portion of the intermediate becomes the cladding region of the polarization maintaining optical fiber, and the stress applying parts of the intermediate become the stress applying regions of the polarization maintaining optical fiber. Moreover, since the stress applying parts and the cladding portion have different thermal expansion coefficients, the residual stresses having non-axisymmetry act on the core region in the manufactured polarization maintaining optical fiber owing to heating at the drawing and cooling after the drawing, and the birefringence is thereby generated.
JP 60-242406A discloses a method of drawing an optical fiber, which maintains a flat plane on a cladding surface, by use of a preform in which a cladding surface on the side without provision of a stress applying part is polished into a flat plane in advance. Meanwhile, JP 2001-220168A discloses a method of manufacturing a multi-mode optical fiber having a core with a noncircular cross section by use of a preform having a noncircular cross section.